Process for the electrodeposition of tin



July 18, 1933. c. J. WERNLUND ET AL 1,919,009

PROCESS FOR THE ELECTHODEPOSITION 0F TIN Filed Feb. 6, 1929 026 05 0526Q57) 04 G425 045 0 5 @525 0.55 Q52! 05 0625 G65 0675 0.7 Momury OFALKHL/Mzmz. Sm/v/vere A TTORNEY}.

a K emcaammummmmmma A V Patented July 18, 1933 UNITED STATES CHRISTIANJOHN WERNLUND, OF TOTTENVILLE, NEW YORK, AND FLOYD I. OPLINGER,

OI PERTH AMBOY, NEW JERSEY,

ASSIGNORS, BY MESNE ASSIGNMENTS, TO E. I. DU

PONT DE NEHOUBS AND COMPANY, A CORPORATION OF DELAWARE PROCESS FOR THEELECTRODEPOSITION OF TIN ,Applicatlon filed February 8, 1929. Serial No.887,824.

This invention has for its object the continuous production of white,smooth and non-porous tin deposits over extended periods of time and ata high rate of speed, with practically no production of waste materials.

In a copending application, Ser. No. 337 833, filed of even dateherewith by one of us, F. F. Oplinger, and issued as U. S. Patent No.1,841,978, it is shown that highly satisfactory deposits of tin may beproduced from alkaline tin baths containing caustic alkali and alkalimetal stannate and in which the analytically determined concentration ofalkali metal stannate bears a definite relationship to the analyticalldetermined alkalinity. The alkalinit 0 these baths, for eachconcentration of a kali metal stannate, may vary a limited amount aboveor below an optimum value as described more fully below.

Under ideal lating conditions, such baths will give good tin depositscontinuously. But, in practical work and when operating in a largescale, the plating conditions are seldom ideal and in time it may benecessary to adjust the bath composition, as for example, by addingeither caustic alkali or alkali metal stannate, in order to keep theconcentration relationship within the limits specified. If one is usinga bath in which the alkalinity has become so high as to exceed therelationship referred to above, the addition of alkali metal stannate,in order to increase the relative amounts of the latter and thereforedecrease the relative alkalinity, may lead to serious difliculty in thatthe added alkali metal stannate when dissolved in the water of thesolution produces an additional alkalinity of its own. The totalalkalinity of the solution is often further increased, when usingcommercial alkali metal stannate, b the presence of free caustic in thesame. ince the addition of large amounts of alkali metal stannate onlysli htl increases the ratio of its concentration in t e solution ascompared with the resulting alkalinity, it is obvious that this methodof adjusting will soon fill the baths with large quantities of dissolvedmaterial and is uneconomical and impractical, particularly at higherconcentrations of alkali metal stannate and at concentrationsafiproaching saturation of the bath with this sa t.

If one attempts to reduce the alkalinit of an alkaline tin plating bathcontaining al ali metal stannate and caustic alkali by adding acidalone, the bath becomes inoperative, since precipitates and sludges areformed and further deposition of tin will be in the form of spongycoatings.

We have now found, that the alkalinity of a tin lating bath containingcaustic alkali and alliali metal stanate can be reduced, without harm tothe bath, by adding to the latter an acid, preferably a weak acid, suchas acetic acid, and at approximately the same time adding an oxidizingagent, such as roxides, permanganates and persulfates o alkali metals,or hydrogen peroxide or alkali metal erborate. N o precipitates orsludges are ormed in the bath when the oxidizin agent is used inconnection with the acid. e refer the use of'hydrogen peroxide or a alimetal perborate. The amounts of oxidizing agent added at any one timewill usually be small, as for example, about 1 to 3 ounces of 25 volumehydrogen peroxide or its equivalent per gallon of plating solution.However, we do not wish to be limited to these specific amounts, sinceany quantity may be used as long as the solution is capable ofdissolving the same. When using hydrogen peroxide, a cold solution willpermit the addition of larger amounts than a hot solution, since thelatter will decompose the hydrogen peroxide more rapidl and will causeviolent oaming if too much is added at one time.

Although we prefer to use acid and oxidizing agent to regulatealkalinity, we have further ound that the operation of alkaline tinplating baths can be improved by the addition thereto of the abovementioned oxidizing agents or per compounds alone, without the additionat the same time of any acid. We do not know what action occurs in thebaths when the oxidizing agent alone is added, but we have noticed thatthe tendenc in baths whose alkalinity is relatively high to depositspongy tin at the cathode, disappears upon the addition of suchoxidizing agents. We believe the action to be that of oxidizing the samecomposition.

any alkali metal stannite which may be present to stannate, therebyslightly increasing the stannate concentration, i. e. reducing therelative alkalinity. The amount of stannite in the solution is probablysmall at all times, since we have found that baths having the properrelationship between alkalinity and stannate concentration for goodplating results do not show the presence of any significant amounts ofstannous tin.

The improvement in the operation of these tin baths, which results whenadding oxidizing agent alone, sometimes disappears after about 30minutes. This is more often the case when the caustic concentration hasbecome too high. However, if one merely adds oxidizing agent from timeto time, the baths will function continuously with the improved results.

We have also found, that baths, having had their composition regulatedby adding to them an oxidizing agent or per salt, will operate forgreater lengths of time without need of adjustment than baths ofotherwise Furthermore, the deposits from baths which have been soregulated appear smoother and brighter than deposits from baths whichhave not been so regulated. We have therefore discovered, that novel tinplating baths which late tin even more satisfactorily than the aths ofthe above mentioned copending application may be produced by firstadding to the latter, small amounts of the oxidizing agents or percompounds used by us in controlling the composition of alkaline tinplating baths.

Our broad invention and a specific application of the same may beexplained more fully by reference to the accompanying graph, which showsthe relationship between the analytically determined alkalinity andalkali metal stannate concentrations in the baths of the above mentionedcopending application and which relationship also holds for our novelbaths containing small amounts of oxidizing agents or per compounds, asspecified above. This relationship may be maintained by the presentinvention for each concentration of alkali metal stannate within thelimits of alkalinity shown between the lines A and C.

The alkalinity is designated on the graph as caustic molality and wasdetermined by analysis, since determinations of alkalinity by othermethods will be affected by various factors, such as impurities usuallypresent in commercial alkali metal stannate, among which are freecaustic, the natural alkalinity of an aqueous solution of alkali metalstannate, etc. The alkali metal stannate content is also determinedanalytically for similar reasons and is designated on the graph asmolality of alkali metal stannate.

To simplify further explanations of our invention, we wish whereverreferring to concentrations of caustic alkali or, specifically, sodiumhydroxide, whether stated in terms of normality, molality, ounces pergallon, or the like, to be understood as referring, not toconcentrations of caustic added as such to the bath, but to totalalkalinity, as determined by titration, using thymol hthalein as anindicator or using an equiva ent procedure. Likewise, wherever referringto concentrations of alkali metal stannate, we wish it to be understoodas referring to those concentrations determined either by an analysis ofthe solution itself or by making up the bath with a material thestannate content of which is accurately known.

In determining caustic alkali concentration, thymol phthalein is used asan indicator, since its blue color in the presence of caustic alkali andalkali metal stannate mixtures disappears upon the addition of acid, forexample of hydrochloric or sulfuric acid, at the same time thatinsoluble tin compounds begin to precipitate. Of course, otherindicators can be used instead of thymol phthalein, provided onecorrects the concentration of caustic alkali so determined to give itthe same value as that which would be obtained by the use of thymolphthalein. Such corrections could be made on the basis of pH values ofthe solution.

Referring again to the accom anying graph, the concentration of causticor optimum plating results, as compared with the concentration of alkalimetal stannate, is represented by the line B of the graph and 7 may alsobe represented by the equation:

Caustic alkali (normality)=0.4 times the alkali metal stannate(normality) +0.3.

In this equation, a normal solution of alkali metal stannate isconsidered as containing 4 mole of stannate per liter. The aboveequation may therefore be expressed as,

Caustic molality=1.6 times the stannate molality 0.3.

The range between lines A and C represents the values for ratio ofcaustic to stannate concentration of our alkaline tin lating bathscontaining oxidizing agents an of the baths of the copending applicationby F. F. Oplinger which do not contain these oxidizing agents. Outsideof this range, less advantageous deposition of tin results. The

range between lines A and C also represents values for caustic alkaliconcentration in the above equation when the figure 0.3 is substitutedby values between 0.2 and 0.4. The figure 0.3 in the two equationsdenotes the concentration of 0.3 moles per liter of caustic which isalways present in these baths at optimum plating compositions,regardless of the stannate concentration.

In carrying out our process of regulating the composition of alkalinetin plating baths we can apply it specifically to maintaining therelationship shown between the lines A and C of the graph. We have foundas a rough shop test, that a thin-film of light greenish-yellow colorexists during electrol sis on the anode when the bath contains t eproper caustic concentration. When electrolysis is discontinued theanode becomes clearer and brighter under the same conditions. When toomuch caustic is present, the anode is clean during the electrolysis andhas the ordinary dull appearance of tin in caustic solution. A whitishto yellowish coating forms on the anode when the caustic content is toolow. The observations on the condition of the anodes can be followed inpractical work by the addition of moderate quan tities of caustic or ofthe oxidizing agents which we have specified, depending upon whether thesolution is low or high respectively in caustic. A weak acid, such asacetic, may be added along with the oxidizing agent. 4

As an improvement over this more or less qualitative control, we preferto analyze the solution for caustic as given above and also forstannate, and then to add the calculated quantity of materials such ascaustic alkali, alkali metal stannate, oxidizing agent, or oxidizingagent and weak acid, in order to bring the bath composition nearer tothe optimum value represented by line B of the graph.

We shall now further describe our alkaline tin plating baths containingoxidizing agents. Our preferred solution and method of operation are asfollows:

Example I 0 2 ingredients by analysis .4.0 to Temperature of solutlon toC. (158 to 176 F.)

Example I I Sodium stannnte 0.634 moi/l. or 18.0 oz/gal.

Sodium hydroxide 1.3 moi/l. or 6.94 oz/gai.

(Concentrations of the above 2 ingredients by analysis of solution)Hydrogen peroxide (25 volume) l/15 oz/gal. Anodes Straits tin Ratio ofanode to cathb de c 5:33 "F5 "'12" o i 60 A s a e curren ens y--- o F E.M F 4.0 to 6.0 volts Temperature of solution--- 70 to 80 C. (158 to 176F.)

Other examples of our baths may be ohtained by consulting theaccompanying graph wherein s mbol 0 denotes com ositions from whicexcellent deposits were 0 tained; the anodes remaining clean and theanode and cathode efliciencies being equal to within about 10%. Thesymbol at on the graph above line A denotes bath compositions from whichspongy deposits were obtained. Various quantities of oxidizing agents,such as peroxides, permanganates, persulfates, perborates, hydrogenperoxide, etc. were used in each of the baths shown on the gra h, butthese quantities were generally sma 1.

Instead of making up our tin baths with alkali metal stannate itself, wemay use any tin salt, which does not put objectionable impurities in thebath in the presence of caustic alkali and the oxidizing agent used, toform the stannate in situ.

The sodium stannate concentration of our baths may vary from practicallyzero to the value at saturation of the solution with the same; goodresults being obtained between 0.07 and 0.70 moles per liter or betweenabout 2.0 and 20 oz. per gallon, provided the caustic soda contentalways bears a close relation to the stannate content as shown by thegraph. Our preferred range, however, lies between about 0.25 and 0.65moles per liter or, between about 7.0 oz/gal. and 18.5 ozs. per allon,since experience has demonstrated that elow this range of stannateconcentration, too much or too little caustic, to fall in the rangerepresented between lines A and C, is much more likely to be producedduring plating as the result of accidental variations in conditions,such as changes in current density, temperature, introduced impurities,etc. Below 7 oz/gal. of sodium stannate, or its equivalent in otheralkali stannate, the current efliciencies are usually lower with anattendant poor quality of deposit. On the other hand, when the stannateconcentration is greater than about 18.5 oz-gaL, the losses produced bythe adhering of some of the solution to the plated articles as they areremoved from the bath is too great for economical operation.Cyrstallization of tin salts from solutions at these higherconcentrations has also been noticed.

Good results may be obtained by keeping the plating solution below C.,but we prefer the range of 60 to 80 C. Above 85 C. there is danger ofprecipitation of solids within the bat \Ve have found that our tin bathsoperate satisfactorily with cathode current densities above about 10amperes per square foot, good deposits having been produced at A/SFSatisfactory anode current densities lie below 20 A/SF. Our tin platingbaths may be used for plating many kinds of metal and particularlysteel, copper, brass, lead, zinc, cadmium, cast iron and the like.

' What we claim is:

1. Process for diminishing the ratio of caustic alkali to alkali metalstannate concentration in an aqueous tin plating solution containingthese substances, comprising adding to said solution an acid and anoxidizing agent of the group consisting of hydrogen peroxide, peroxides,permanganates, persulphates, and perborates.

2. Process for diminishing the ratio of caustic alkali to alkali metalstannate concentration in an aqueous tin plating solution containingthese substances, comprising adding to said solution a weak acid and anoxidizing agent of the group consisting of hydrogen peroxide, peroxides,permanganates, persulphates, and perborates.

3. Process for diminishing the ratio of caustic alkali to alkali metalstannate concentration in an aqueous tin plating solution containingthese substances, comprising adding to said solution a Weak acid and aper compound.

4. Process for diminishing the ratio of caustic alkali to alkali metalstannate concentration in an aqueous tin plating solution containingthese substances, comprising adding to said solution a weak acid andhydrogen peroxide.

5. Process for diminishing the ratio of caustic alkali to alkali metalstannate concentration in an aqueous tin plating solution containingthese substances, comprising add ing to said solution acetic acid and anoxidizing agent of the group consisting of hydrogen peroxide, peroxides,permanganates, persulphates, and perborates.

6. Process for diminishing the ratio of caustic soda to sodium stannateconcentration in an aqueous tin plating solution containing thesesubstances, comprising adding to said solution a Weak acid and anoxidizing agent of the group consist ng of hydrogen peroxide, peroxides,permanganates, persulphates, and perborates.

7. Process for diminishing the ratio of caustic soda to sodium stannateconcentration in an aqueous tin plating solution containin thesesubstances, comprising'adding to said solution acetic acid and anoxidizing agent of the group consisting of hydrogen peroxide, peroxides,permanganates, persulphates, and perborates.

8. Process for diminishing the ratio of caustic soda to sodium stannateconcentration in an aqueous tin plating solution containing thesesubstances, comprising adding to said solution acetic acid and a rcompound.

9. Process for diminis ing the ratio of caustic soda to sodium stannateconcentration in an aqueous tin plating solution containing thesesubstances, comprising adding to said solution acetic acid and hydrogenperoxide.

10. A process for the electrodeposition of metallic tin, comprisingconducting an electrolyzing current from a tin' anode to the article tobe coated as a cathode through an aqueous solution of alkali metalstannate,

caustic alkali and a smallamount of a r compound, the composition ofwhich solutlon is limited by a caustic molality which increases at auniform rate from 0.7 when the stannate molality is-0.25, to 1.34 whenthe stannate molality is 0.65, the range of caustic molality at eachmolality of stannate'being 0.1 less to 0.1 more than the values thusdefined.

11. A process for the electrodeposition of metallic tin, comprisingconducting an electrolyzing current from a tin anode to the article tobe coated as a cathode through an aqueous. solution of alkali metalstannate, caustic alkali and a small amount of a r compound, thecomposition of which solution is limited by a caustic molality whichincreases at a uniform rate from 0.7 when the stannate molality is 0.25,to 1.34 when the stannate molality is 0.65, the range of causticmolality at each molality of stannate being 0.1 less to 0.1 more thanthe values thus defined and adding further quantities of said percompound from time to time.

12. A process for the electrodeposition of metallic tin, comprisingconducting an electrolyzing current from a tin anode to the article tobe coated as a cathode through an aqueous solution of sodium stannate,caustic soda and a small amount of a per compound, the composition ofwhich solution is limited by a caustic molality which increases at auniform rate from 0.7 when the stannate molality is 0.25 to 1.34 whenthe stannate molality is 0.65, the range of caustic molality at molalityof stannate being 0.1 less to 0.1 more than the values thus defined.

13. A process for the electrodeposition of metallic tin, comprisingconducting an electrolyzing current from a tin anode to the article tobe coated as a cathode through an aqueous solution of sodium stannate,caustic soda and a small amount of a per compound, the composition ofwhich solution is limited by a caustic molality which increases at auniform rate from 0.7 when the stannate molality is 0.25 to 1.34 whenthe stannate molali't is 0.65, the range of caustic molality at mola ityof stannate being 0.1 less to 0.1 more than the values thus defined andadding further quantities of said per compound from time to time.

14. An aqueous electrolyte for the electrodeposition of tin, containing1.6 moles of canstic alkali per mole of alkali metal stannate and anadditional concentration, not dependent upon the amount of saidstannate, of about 0.2 to 0.4 moles of caustic alkali per liter, plus asoluble per compound.

15. An aqueous electrolyte for the electrodeposition of tin, containing1.6 moles of caustic alkali per mole of alkali metal stannate and anadditional concentration, not dependent upon the amount of saidstannate, of about 0.2 to 0.4 moles of caustic alkali per liter, plushydrogen peroxide.

16. An aqueous electrolyte for the electrodeposition of tin, containing1.6 moles of caustic alkali per mole of alkali metal stannate and anadditional concentration, not dependent upon the amount of saidstannate, of about 0.2 to 0.4- moles of caustic alkali per liter, theconcentration of alkali metal stannate being 0.25 to 0.65 moles perliter, plus a soluble per compound.

17 An aqueous electrolyte for the electrodeposition of tin, containingabout 1 mole of caustic alkali and about 0.4375 moles of alkali metalstannate per liter, plus a soluble per compound.

18. An aqueous electrolyte for the electrodeposition of tin, containingabout 1 mole of caustic alkali and about 0.437 5 moles of alkali metalstannate per liter, plus hydrogen peroxide.

19. An electrolyte for the electrodeposition of tin, comprising anaqueous solution conganates, persulphates, and perborates to saidsolution from time to time.

21. In the process of electrodeposition of tin from a caustic soda andsodium stannate bath, the step which comprises adding a per compound tosaid solution from time to time.

22. In the process of electrodeposition of tin from a caustic soda andsodium stannate bath, the step which comprises adding hydrogen peroxideto said solution from time to time.

CHRISTIAN JOHN WERNLUND. FLOYD F. OPLINGER.

